This proposal is focused on elucidating the biochemical basis of aberrant DNA synthesis in the dividing and non-dividing cells in eucaryotic organisms, with strong emphasis placed on the study of brain cells. Somatic mutations are an important source of human disease. In addition to mutations occurring in dividing cell populations, there is accumulating evidence that mutations are also occurring in non-dividing cells, both procaryotic and eucaryotic. Genomic DNA is constantly undergoing potentially deleterious modifications in the form of slippage expansions and deletions, deaminations, loss of bases, UV damage, and chemically-induced adduct formations, to cite just a few examples. All living organisms have evolved powerful, highly selective biochemical pathways to repair damaged DNA. However, these repair processes are not perfect, and indeed they cannot be, because living organisms must be able to mutate in order to adapt to changing local environments and to evolve. Our proposal addresses questions of how eucaryotic cells copy DNA containing unrepaired lesions. There is recent evidence for an alternative pathway to apoptosis (programmed cell death), involving "error-prone" eucaryotic proteins required to alleviate blocked DNA synthesis at lesion sites and to catalyze lesion bypass leading to mutations. Our specific aims re to identify, purify and characterize eucaryotic lesion bypass proteins.